The voice of Seth Briars explains how he got the part

Kevin Glikmann, starts out with a call from his agent to audition for a company known for casting commercial (Kathy Knowles Casting). He essentially gets surprised with a crazy grave robber/gollum type roll and nails it with absolute crazy acting. 3 months later BOOM he has the role of Seth Briars voice in Red Dead Redemption.

That's my short version of the story. Feel free to read it straight from him on reddit.